The present invention relates to an impedance measuring apparatus and method for measuring impedance for fuel cell diagnosis, and more particularly, to an apparatus and method for measuring impedance for fuel cell diagnosis, which monitors a current flowing through a load, and performs frequency analysis between sampling cycles while sampling the current and voltage of a fuel cell stack, when the current flowing through the load is varied by a preset value or more, thereby shortening a time required for separate frequency analysis after sampling and reducing the number of separate current injection circuits.
In general, a fuel cell is a kind of generator which does not convert chemical energy of fuel into heat through combustion, but converts the chemical energy into electrical energy through an electrochemical reaction inside a stack.
Such a fuel cell may not only supply power for industrial use, power for domestic use, and power for vehicle operation, but also supply power to small-sized electric/electronic products.
For example, much research has been conducted on PEMFC (Polymer Electrolyte Membrane Fuel Cell or Proton Exchange Membrane Fuel Cell) having the highest power density among fuel cells serving as power supply sources for vehicle operation. Since the PEMFC has a low operating temperature, the PEMFC has a fast starting time and a fast power conversion reaction time.
The PEMFC includes an MEA (Membrane Electrode Assembly), a GDL (Gas Diffusion Layer), a gasket and coupling mechanism, and a bipolar plate. The MEA has a catalyst electrode layer attached at both sides of a solid polymer electrolyte membrane through which hydrogen ions are moved, and an electrochemical reaction occurs in the catalyst electrode layer. The GDL serves to uniformly distribute reaction gases and transmit generated electrical energy. The gasket and coupling mechanism serves to maintain a sealing property for reaction gases and cooling water and a proper clamping pressure. The bipolar plate serves to move the reaction gases and cooling water.
When such a unit cell configuration is used to assemble a fuel cell stack, a combination of the MEA and the GDL is positioned at the innermost part of the unit cell. The MEA has a catalyst electrode layers formed on both surfaces of the polymer electrolyte membrane, that is, an anode and cathode, the catalyst electrode layers being coated with a catalyst which can react with hydrogen and oxygen. Outside the anode and cathode, the GDL and the gasket are stacked.
Outside the GDL, the bipolar plate is positioned. The bipolar plate has a flow field to which reaction gas is supplied and through which cooling water is passed, the reaction gas containing hydrogen serving as fuel and air or oxygen serving as an oxidizer.
Such a configuration is set to a unit cell. After a plurality of unit cells are stacked, a current collector, an insulating plate, and an end plate for supporting the stacked cells are coupled to the outermost part. Between the respective end plates, the unit cells are repetitively stacked and coupled to form the fuel cell stack.
In order to obtain a potential required by an actual vehicle, a plurality of unit cells corresponding to the required potential must be stacked. The unit cells are stacked to form a fuel cell stack.
The potential generated from one unit cell is about 1.3V. For example, a plurality of cells is stacked in series to produce power required for driving the vehicle.
Since a fuel cell used for a vehicle requires stability, failure diagnosis needs to be accurately and rapidly performed.
Examples of the method for diagnosing a fuel cell may include a cell voltage measurement method, EIS (Electrochemical Impedance Spectroscopy), a nonlinear response method, and a current interrupt method.
The cell voltage measurement method measures the voltages of all cells forming a fuel cell stack, and determines that a failure occurred when one or more cells and another cell have a large potential difference therebetween.
The EIS changes a frequency through a method of calculating an internal parameter of a system model, and uses impedance obtained by measuring an output voltage based on a small AC current.
The nonlinear response method diagnoses a failure by determining the linearity of a fuel cell system model. Similar to the EIS, the nonlinear response method measures an output voltage based on an AC current, calculates a high-frequency distortion factor of the measured voltage, and determines the linearity of the system model, in order to diagnose a failure.
The current interrupt method passes a constant current to a current cell, and instantly interrupts the current. Then, the current interrupt method calculates internal resistance based on the voltage change.
Among the methods, the cell voltage measurement method can accurately distinguish between abnormal cells and normal cells, because the cell voltages of all cells within the fuel cell stack are monitored. However, since a fuel cell stack for a vehicle includes about 400 or more cells, the configuration may become complex and the price competitiveness and reliability thereof may be degraded, when all of the cells are connected to a harness in order to measure voltages.
The nonlinear response method has a simpler system configuration than the cell voltage measurement method. However, the nonlinear response method requires complex operations during failure diagnosis through harmonic analysis for an output voltage, and requires complex control and much cost when generating a sine-wave alternating current.
Furthermore, the EIS, the current interrupt method, and the nonlinear response method change the state of a fuel cell by injecting or attracting a specific voltage/current to or from the fuel cell. Thus, the EIS, the current interrupt method, and the nonlinear response method may have difficulties during vehicle operation, and affect the operation and performance of the fuel cell.
The related art of the present invention is disclosed in Korean Patent Laid-open Publication No. 10-2014-0085802 published on Aug. 8, 2014 and entitled “Method and system for measuring impedance for state diagnosis of fuel cell stack”.